1. Field of the Invention
The invention relates to a lifting device, viz. for a vehicle, comprising, one the one hand, a substantially vertical column forming a frame and lifting means movable alongside said column under the action of the rotation control, through appropriate driving means, of a threaded rod and the movement alongside this latter of a reversible-type nut, such as a ball nut made of polyamide, compound material or the like, and, on the other hand, friction means capable of creating, during the control of the down movement of the lifting means, a resistance torque of a modulus at least equal to the torque generated by the load resting on these lifting means, as well as means capable of annihilating the action of the friction means during the control of the up movement of said lifting means.
2. Description of the Prior Art
With a view to carrying out repairs on light motor vehicles, there exist lifting ramps onto which vehicles can be positioned. However, these lifting ramps generally are not feasible for larger-size and heavier-weight vehicles, e.g., a large vehicle for transport of goods or the public or even one or several railway vehicles. In such cases, lifting devices corresponding to the above description are often used. More particularly, these lifting devices are in the shape of a vertical column forming a frame and resting on the ground, through a base. It should be noted that the base is usually provided with rollers allowing movement of the lifting device and thus enabling bringing the same device into the vicinity of the vehicle to be lifted. Alongside this vertical column are moving lifting arms in the shape of one or several horizontal arms, each associated with a movable carriage. The movable carriage is guided in movement alongside the column through guiding rails and provided with a set of rollers. As regards the control of the movement of the carriage, this is achieved especially through a vertical threaded rod rotatingly fitted inside the column and capable of being driven by driving unit, e.g., an electric engine, situated at the top of the column. Furthermore, a nut made integral with the lifting means is fitted onto the threaded rod so that during the rotation control of the rod the nut moves up or down, thereby driving the lifting device.
It is obvious that these lifting devices for lifting relatively heavy loads include a number of safety devices impeding the load from moving down during any failure, e.g., of the driving unit.
More particularly, within the framework of a normal operation these driving units provide a resistance torque largely higher than the torque exerted onto the threaded rod through the lifted load. Thus, unless the operation of these driving means is controlled, this load can in no way move down. This is obviously no longer so in the event of a failure of these driving units. For this purpose, there is often used a nut, e.g., made of bronze, the spiral line of the thread pitch being determined so that it leads to a resistance torque in particular higher than the torque generated during the down movement by the lifted load. Such a nut is called irreversible.
Thus, though this irreversible nut meets the above-mentioned requirements, it has notwithstanding a number of drawbacks in that it leads to a low efficiency and a high power absorption as well as heating power generation. Therefore, the driving unit should be of an accordingly high power. Furthermore, the use of a lubricating system is absolutely necessary.
As a result, these kinds of lifting devices with an irreversible nut are of a high cost price, especially because of the presence of a powerful electric engine. It should be noticed by the way that the electric-energy supply of these powerful electric engines may give rise to problems when lifting a large-size vehicle, e.g., of the railway-vehicle type, requiring the use of several lifting devices. Since these lifting devices are necessarily simultaneously controlled, they do indeed imply a large electrical-energy requirement.
An alternative to bronze irreversible nuts is reversible nuts with a high efficiency, such as ball nuts or nuts made of polyamide or compound material. Of course, their reversibility brings the drawback of an inexisting safety in the event of failure of the driving means. More particularly, in such circumstances, the torque generated by the load is largely higher than the resistance torque of the reversible nut. As a consequence, this results into almost instantaneous moving down of the lifting device and the fall of the load.
In order to cope with these kinds of drawbacks, it has been contemplated to fit the lifting device with friction devices capable of creating, during the control of the down movement of the lifting means, a resistance torque capable of opposing the torque generated by the load resting on these lifting devices. However, devices capable of overcoming the action of the friction devices are required during the control of the up movement of the lifting devices.
More particularly, the threaded rod is hanging at its upper end, through a bearing block located on top of the column. This bearing block includes a casing defining a cylindrical recess into which penetrates in particular this upper end of the threaded rod. The threaded rod is provided, at this level, with a shoulder resting onto a thrust ball bearing which, in turn, rests on the outer cage of a free wheel arranged inside the cylindrical recess defined at the level of the bearing block.
In fact, this free wheel forms the device capable of overcoming the actions of the friction devices during the control of the up movement of the lifting means. More particularly, this free wheel fitted onto the upper end of the threaded rod leads to the driving of this outer cage only when the threaded rod has penetrated in the rotating direction, causing the lifting means to move down. As a result, in the opposite rotation direction, the outer cage of this free wheel is not driven, whereby the same is impeded from causing the friction devices to act. The friction devices are in the shape of friction discs arranged on the bottom of the bearing block and onto which rest the outer cage of the free wheel.
In fact, the drawback of such a construction resides in that, irrespective of the lifted load, it fully rests on the friction device, through the cage of the free wheel. Thus, when the lifting devices bear a heavy load, these friction devices are highly stressed, so that they quickly wear off and lead to a considerable heating.